A filter rod of this type includes a cylindrical filter material and paper wrapping the filter material. The filter material is formed, for example by a machine disclosed in Japanese Unexamined Patent Publication No. Hei 7-203935.
Specifically, the machine disclosed in the above publication comprises a tow treatment section for forming a flat sheet-like filter web from fibrous tow, and a forming section for receiving the filter web from the tow treatment section and gathering up the received filter web to form it into a rod-shaped material.
The rod-shaped material is then fed from the forming section to a wrapping section. While passing through the wrapping section, the rod-shaped material is wrapped in a paper web and formed into a filter rod continuum. The filter rod continuum is cut into individual filter rods.
The filter rods thus formed are, for example, cut to a predetermined length and used as filter tips for cigarettes. When a filter cigarette is smoked, the filter tip captures nicotine and tar contained in mainstream smoke of the filter cigarette and serves to lighten the feeling that the filter cigarette gives when smoked.
When the filter tip contains a granular additive, for example activated carbon grains, the activated carbon grains absorb undesired substances contained in mainstream smoke and serves to improve the taste of a filter cigarette.
The filter tip containing such additive grains is called a dual filter tip. The dual filter tip comprises a plain half-tip and a functional half-tip. The plain half-tip is obtained by cutting the above-mentioned plain filter rod, while the functional half-tip is obtained by cutting a functional filter rod containing additive grains.
In order to obtain such a functional filter rod, it is conceivable to arrange an additive scattering device above between the tow treatment section and the forming section, and scatter additive evenly over the filter web from the scattering device.
However, in this arrangement, the filter web is completely free on a transport path thereof from the terminal end of the tow treatment section to the forming section. Due to the weight of the filter web and the weight of the additive scattered over the web, the filter web bends downward in its widthwise central part, and is deformed to an arched transverse cross-section. Further, while travelling, the filter web oscillates vertically.
This deformation and oscillation of the filter web makes the additive scattered over the filter web move to the bottom of the arched filter web, so that the additive is not evenly distributed over the filter web. When a functional filter rod is formed from the filter web with the additive unevenly distributed, the functional filter rod or functional half-tips obtained therefrom do not show uniform additive-distribution-density in the cross-section thereof so that the additive is evenly distributed in the functional half-tips.
In this case, if the additive grains are activated carbon grains, the activated carbon grains are not well exposed to mainstream smoke, and the grains cannot effectively absorb substances contained in the mainstream smoke. As a result, the absorption performance of the dual filter tip lowers.
When the functional half-tip is formed by cutting the functional filter rod, uneven distribution of additive grains increases the amount of the additive grains falling off a cut face of the functional filter rod or the half-tip. Further, if additive grains are located near a lap seam of the paper web in a concentrated manner, additive grains easily enter the lap seam and causes defective wrapping of the paper web. The defective wrapping of the paper web causes a shutdown of a functional-filter-rod making machine and lowers productivity in making the functional filter rods.
In order to improve the performance of the functional half-tip, it is conceivable to increase the amount of additive grains added. However, this further increases the incidence of the above problems, namely additive grains falling and defective wrapping of the paper web.
A machine similar to that described is known. The known machine comprises a guide arranged between the tow treatment section and the forming section to guide the filter web while the web is gathered up. The guide is in the form of a trough having a U-shaped cross-section or a coil tunnel, for example.
However, the guide in either form is not effective in preventing the granular additive from moving on the filter web.